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To monitor and characterize oseltamivir-resistant (OR) pandemic (H1N1) 2009 virus with the H275Y mutation, we analyzed 4,307 clinical specimens from Japan by neuraminidase (NA) sequencing or inhibition assay; 61 OR pandemic (H1N1) 2009 viruses were detected. NA inhibition assay and M2 sequencing indicated that OR pandemic (H1N1) 2009 virus was resistant to M2 inhibitors, but sensitive to zanamivir. Full-genome sequencing showed OR and oseltamivir-sensitive (OS) viruses had high sequence similarity, indicating that domestic OR virus was derived from OS pandemic (H1N1) 2009 virus. Hemagglutination inhibition test demonstrated that OR and OS pandemic (H1N1) 2009 viruses were antigenically similar to the A/ California/7/2009 vaccine strain. Of 61 case-patients with OR viruses, 45 received oseltamivir as treatment and 10 received it as prophylaxis, which suggests that most cases emerged sporadically from OS pandemic (H1N1) 2009, due to selective pressure. No evidence of sustained spread of OR pandemic (H1N1) 2009 was found in Japan; however, 2 suspected incidents of human-to-human transmission were reported. [ABSTRACT FROM AUTHOR]

To monitor oseltamivir-resistant influenza viruses A (H1N1) (ORVs) with H275Y in neuraminidase (NA) in Japan during 2 influenza seasons, we analyzed 3,216 clinical samples by NA sequencing and/or NA inhibition assay. The total frequency of ORVs was 2.6% (45/1,734) during the 2007-08 season and 99.7% (1,477/1,482) during the 2008-09 season, indicating a marked increase in ORVs in Japan during 1 influenza season. The NA gene of ORVs in the 2007-08 season fell into 2 distinct lineages by D354G substitution, whereas that of ORVs in the 2008-09 season fell into 1 lineage. NA inhibition assay and M2 sequencing showed that almost all the ORVs were sensitive to zanamivir and amantadine. The hemagglutination inhibition test showed that ORVs were antigenetically similar to the 2008- 09 vaccine strain A/Brisbane/59/2007. Our data indicate that the current vaccine or zanamivir and amantadine are effective against recent ORVs, but continuous surveillance remains necessary. [ABSTRACT FROM AUTHOR]

Abstract: The sudden emergence of the pandemic influenza A (H1N1) 2009 virus in early 2009 has resulted in a rapid transmission of this virus worldwide. Within a short time span, sporadic cases infected with this virus that shows oseltamivir resistance have also been reported. These resistant viruses have an amino acid change from histidine to tyrosine at position 275 (H275Y) of the neuraminidase gene. In this study, a reverse transcriptase PCR/restriction fragment length polymorphism (RT-PCR/RFLP) assay was developed to detect the H275Y mutation. Resistant and sensitive viruses could be differentiated using the RFLP patterns. This RT-PCR/RFLP assay is a simple method and also very specific and sensitive for detecting the H275Y mutation of pandemic influenza A (H1N1) 2009 viruses, and can be used in resource-limited settings. [Copyright &y& Elsevier]